When I try to download the latest version, the web pages shows 403 Forbidden, ngix.

Wait a little bit, probably a small trouble on the server.
try installing through softaculous or any one script installer
Im going to upgrade an existent board so I cannot relay with softaculous

Hi,
We've recently been having a little trouble with the server. As such, you may sometimes get a 403 error. We're just waiting for a staff member with server access to take a look and fix the issue.
Until then, .m. has posted a media fire link to the most recent version here:
http://community.mybb.com/thread-167911-...pid1144474
Thanks
Tom